This grant aims to study the functional significance of sex differences in neuropeptide systems in the brain. The hypothesis driving this grant is that sex differences in the brain do not only serve to generate sex differences in centrally regulated processes and behaviors, but may also enable males and females to show remarkably similar behaviors, even though their physiological and hormonal conditions differ dramatically. The grant focuses on the role of the sexually dimorphic vasopressin innervation of the brain in parental behavior as it is displayed by two species of voles: prairie voles (Microtus ochrogaster), a monogamous species in which males and females provide parental care, and meadow voles (Microtus pennsylvanicus), a promiscuous species in which only females provide parental care. This strategy allows the comparison of the vasopressin innervation between males and females as well as between males that show different levels of parental involvement. The research will include psychopharmacological, hormonal and developmental manipulations that are known to influence vasopressin neurotransmission, or parental behavior, or both. The consequences of such manipulations will be analyzed at an anatomical, cell biological, and behavioral level. By studying the neural basis of paternal behavior, this study develops a new theme in the field of behavioral neuroendocrinology, because virtually nothing is known about neural structures underlying paternal behavior in mammals. In addition, by showing that different neuropeptide systems may be used to serve similar functions in males and females, this study may affect drug therapies that are based on interference with neurotransmission in the brain, because it underscores the notion that these therapies should be developed independently for men and women.